The present invention relates to form feeding tractors and more particularly, to drive assemblies for tractors which are intended for high speed incremental advance of the form through the tractor.
In the past two decades, there has been rapidly increasing usage of form feeding tractors in printers and other devices which utilize webs such as continuous paper, individual paper sheets, tape and the like. In many instances, the webs have perforations along the side thereof in which drive pins of the tractor engage so as to advance the web therethrough. These webs are typically fabricated of paper, although foil, laminates and other sheet materials are also utilized, all collectively referred to hereinafter as "web material".
Generally, the tractors which are most widely employed have a continuous flexible synthetic resin belt with pins on its upper surface extending into the perforations of the web material, and drive teeth or cleats on the inner surface of the belt engage with cooperatively configured grooves or teeth on the sprockets of the tractor, one of which is driven to effect rotation of the belt.
In recent years belts molded from synthetic resin have been most widely employed. In most instances, the drive pins have been integrally molded with the continuous flat portion of the belt. Illustrative of such belt and sprocket assemblies is Seitz U.S. Pat. No. 4,130,230; in this structure the configuration of the belt and sprockets locates the drive pins in areas between the drive teeth which engage with the underlying sprocket. Thus, the belt and sprocket will mesh only in predetermined relative positions of the drive pins relative to the rotational position of the drive sprocket.
Composite belt constructions have also been proposed in order to facilitate fabrication of the belt or to provide greater wear resistance to the drive pins which are abraded by the web material as it moves downwardly and upwardly thereover. One such composite belt construction is shown in Hubbard U.S. Pat. No. 3,825,162 granted July 23, 1974.
More recently, the emphasis on high speed printers which start and stop at extremely rapid speeds as they advance in relatively short increments have produced conditions which increase the tendency for the web material to erode the surface of the drive pins. Thus, there has been a need to fabricate the pins from a more durable material than the flexible resins which are utilized for the continuous flexible belt.
Accordingly, it is an object of the present invention to provide a novel drive assembly for form feeding tractors in which the belt utilizes metal drive pins which are firmly secured in a relatively flexible strap.
It is also an object to provide such a drive assembly which can be fabricated relatively easily from synthetic resin and metal components, each providing its desirable characteristics to the composite structure.
Another object is to provide such a drive assembly in which the belt will flex readily in the area to either side of the drive pins as it moves about the drive sprockets.